1. Field of the Invention
The present invention is directed to a power source providing output power through a compact high frequency transformer or coupling transformer, and more particularly, to a power source for energizing and/or charging hand-held electrical devices such as transmitters of remote controllers for television sets, video recorders or compact disk players, cordless telephones, portable cassette recorders incorporating a rechargeable battery, or for use as an auxiliary power source for a switching power supply.
2. Description of the Related Art
As a power source for hand-held electrical devices, there has been widely utilized an AC power adaptor which energizes and in most cases charges a rechargeable battery incorporated in the devices. The prior AC power adaptor utilizes a power transformer to convert the available fixed-frequency AC main voltage to a DC voltage. In order to make the adaptor compact it has been proposed to use a small-sized core and a thin wire for the power transformer in view that the product of the number of turns of the wire or winding and the cross-sectional area of the core depends on the frequency and the voltage of the available AC main power. Nevertheless, this scheme has been found effective only to a limited extent in compacting the adaptor and has limited reliability. Further, the use of the power transformer inevitably brings about undesired power loss of about 1 Watt due to the core loss of the power transformer. To avoid these problems, there has been proposed a power adaptor of inverter-type which comprises a rectifier providing a DC voltage from the AC main power, an inverter converting the resulting DC voltage to an AC voltage, and an AC-to-DC converter providing a DC voltage for energization of the device. Although the inverter-type power adaptor is effective to considerably reduce the size of a transformer and therefore successfully down-size the power adaptor, there remains a problem that it necessitates a rather complex circuit for controlling a switching element or elements of the inverter and therefore makes the design very costly. In addition, the inverter-type power adaptor requires a relatively great power for controlling the inverter and suffers from a power loss of about 1 Watt due to the switching loss of the inverter, which renders the inverter-type power adaptor less advantageous in view of the power consumption.